1. Field of the Invention
The present invention relates to technology for reducing image defects, such as smeared images and color misregistration, in a multicolor image forming apparatus.
2. Description of the Related Art
As compared to a rotary method which uses a single photosensitive drum and a rotary unit having a plurality of image developing units, a tandem method accelerates the speed of image formation by forming toner images of different colors in parallel using a plurality of image forming stations. On the other hand, as the tandem method uses a plurality of photosensitive members and a plurality of optical devices, it triggers color misregistration and unevenness of color unless correction is made in accordance with variations in attachment of the photosensitive members and the optical devices and mechanical changes that occur over time. The color misregistration and unevenness of color make it difficult to obtain multicolor images with good quality.
Japanese Patent No. 2655603 proposes reduction of color misregistration by forming toner patches of different colors on an intermediate transfer belt, detecting positions of the toner patches with a sensor, and changing the timings at which toner images of different colors are applied to the intermediate transfer belt in accordance with the result of detection. Note that the toner patches denote unfixed toner images used to detect color misregistration.
Incidentally, with the tandem method, when the photosensitive members are always kept in contact with developing rollers, the surface layers of the photosensitive members are worn off by being rubbed against the developing rollers. This shortens the life of the photosensitive members. Japanese Patent Laid-Open No. 2006-323235 proposes a technique to, by rotating a plurality of cams with a single drive source, switch photosensitive members to a developing state sequentially from upstream to downstream, and switch the photosensitive members to a non-developing state sequentially from upstream to downstream.
However, when switching the photosensitive members to a developing state sequentially from upstream to downstream as in the invention described in Japanese Patent Laid-Open No. 2006-323235, color misregistration may be enhanced. This is caused by a difference between the circumferential surface speed of the intermediate transfer belt when detecting positions of the toner patches thereon and the circumferential surface speed of the intermediate transfer belt at the time of image formation. In order to reduce the occurrence of the above color misregistration and image defects, it is necessary to control a difference in the circumferential surface speeds of the photosensitive drums and the intermediate transfer belt, as well as a difference in the circumferential surface speeds of the intermediate transfer belt and a secondary transfer roller, so as to achieve favorable image formation.